If you're trying to upgrade your walls to level 5, you may see a notice that you need to research Ablative Panels III before you can begin the upgrade. Ablative Panels III is often confused for Ablative Armor III, making it easy to believe you have completed the prerequisite research when you have not.
Ablative Armor III is a ship special and can be found in the "Ship Specials" section of your Advanced Lab:
Ablative Panels III is a base special and can be found in the "Base Specials" section of your Advanced Lab:
If you're having trouble upgrading your walls to level 5, be certain that you've researched the correct special!